The present invention relates generally to ignition systems for internal combustion engines and more specifically to ignition coil assemblies used in such ignition systems.
In ignition systems for internal combustion engines, one or more ignition coil assemblies are typically provided. Each assembly typically has a xe2x80x9cprimaryxe2x80x9d coil and a xe2x80x9csecondaryxe2x80x9d coil, these coils being magnetically coupled. Relatively low-voltage electrical energy is switched though the primary coil, inducing higher voltage electrical energy in the secondary coil. This higher voltage is provided to an ignition device such as a spark plug. The higher-voltage energy breaks down an air gap in the spark plug, causing a spark which causes ignition in the engine.
The transient nature of the higher-voltage energy in the secondary circuit of an ignition coil tends to create electromagnetic fields which can be disruptive to electronic devices nearby. These fields are of considerably higher frequency than the frequency at which the inductively sourced energy from the ignition coil is delivered to the engine.
In one known method of trying to reduce the electromagnetic fields, a resistor is placed in the spark plug boot, in contact with the top of the spark plug (that is, in series in the secondary circuit of the ignition coil). Such a resistor cooperates with the inductance already in the secondary circuit (or, in the case of a wire-wound resistor, the resistor adds additional inductance) to filter the frequencies where electromagnetic fields are a concern. Although such a design may be generally effective in reducing electromagnetic fields, the design might not provide sufficient impedance at all frequencies where suppression is desired. Also, a resistor adds impedance not only in frequencies where suppression is desired, but also at lower frequencies, where spark energy is delivered. This reduces the amount of spark energy which is delivered. Further, the resistor suppression design adds additional electrical connections between the spark plug and the secondary ignition coil, adding potential unreliability to the system.
U.S. Pat. No. 3,131,133 uses a suppresser located in the spark plug boot. This is not a stand-alone element for RF filtering. The filtering scheme disclosed in the ""133 patent uses capacitance in the high tension wire to form a filter. Two low pass filters are thus formed in the ""133 patent. In such a design, a non-resistive spark plug is used. It is believed that if a resistive spark plug was used, the filter would be defeated. Also, since the ""133 patent relies on capacitance from high tension wire, it is not compatible with coil-on-plug ignition, which does not use such wires.
U.S. Pat. No. 3,822,341 uses a ferrite-spring installed within the spark plug in the place of the normal resistive element. By placing the ferrite core within the location of the spark plug, a relatively small piece of ferrite is used and therefore high instantaneous current discharges that occur are believed to saturate the ferrite core too quickly. Also, such a design is also believed to have limited applications because the Curie temperature of the ferrite is believed to be reached for most applications. The Curie temperature is reached because spark plugs typically operate between 400 and 900 degrees to allow the ceramic surface to burn any carbon deposits thereon. Also, since the ""341 patent uses the ferrite-spring that occupies the space normally used by a spark plug resistor, it is not possible to use a ferrite-spring combinationally with a spark plug resistor.
Thus, a design which reduces the potential unreliability of additional connections from a resistor element in the spark plug boot while effectively reducing electromagnetic fields and not reducing delivered spark energy will provide advantages over the prior art.
The present invention provides an ignition coil assembly. The ignition coil assembly comprises a coil spring made of electrically conductive material, an electrical coil connected to deliver electrical energy from the ignition coil assembly via the coil spring and a ferromagnetic member disposed within the coil spring.
Devices according to the present invention effectively suppress emitted electromagnetic fields and reduce the unreliability of an added resistor in the spark plug boot. Further, devices according to the present invention can be less expensive than an added resistor in the spark plug boot. The present invention thus provides considerable advantages over alternative designs. A resistive spark plug is coupled to the ignition coil in electrical contact with the coil spring. The resistive element within the spark plug in combination with the ferromagnetic member form a filter that suppresses radio frequency interference.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.